This invention relates to anion exchange chromatography media based on polymeric primary amines, an anion exchange sorber including that media, and a chromatography scheme including the sorber. Absorption refers to taking up of matter by permeation into the body of an absorptive material. Adsorption refers to movement of molecules from a bulk phase onto the surface of an adsorptive media. Sorption is a general term that includes both adsorption and absorption. Similarly, a sorptive material or sorption device herein denoted as a sorber, refers to a material or device that either ad- or absorbs or both ad- and absorbs. The media is particularly applicable as a porous membrane sorber used in a flow through cartridge and more particularly to a cartridge free of a separate exterior housing.
Strong anion exchangers, such as those based on quarternary ammonium ions, are used in downstream processing as a polishing media for capturing negatively charged large impurities, such as endotoxins, viruses, nucleic acids, and host cell proteins (HCP) that are present in fluids such as biological fluids, particularly solutions of manufactured biotherapeutics Traditionally, anion exchangers have been offered and used in the bead format, for example Q Sepharose® available from GE Healthcare Bio-Sciences AB. However, throughput limitations of bead-based systems require large volume columns to effectively capture impurities.
In bead-based chromatography, most of the available surface area for adsorption is internal to the bead. Consequently, the separation process is inherently slow since the rate of mass transport is typically controlled by pore diffusion. To minimize this diffusional resistance and concomitantly maximize dynamic binding capacity, small diameter beads can be employed. However, the use of small diameter beads comes at the price of increased column pressure drop. Consequently, the optimization of preparative chromatographic separations often involves a compromise between efficiency/dynamic capacity (small beads favored) and column pressure drop (large beads favored).
In contrast, membrane-based chromatographic systems (also called membrane sorbers), have the ligands attached directly to the convective membrane pores, thereby eliminating the effects of internal pore diffusion on mass transport. Additionally, the use of microporous membrane substrates with a tight membrane pore size distribution coupled with effective flow distributors can minimize axial dispersion and provide uniform utilization of all active sites. Consequently, mass transfer rates of membrane sorber media may be an order of magnitude greater than that of standard bead-based chromatography media, allowing for both high efficiency and high-flux separations. Since single or even stacked membranes are very thin compared to columns packed with bead-based media, reduced pressure drops are found along the chromatographic bed, thus allowing increased flow rates and productivities. The necessary binding capacity is reached by using membranes of sufficient internal surface area, yielding device configurations of very large diameter to height ratios (d/h).
Properly designed membrane sorbers have chromatographic efficiencies that are 10-100 times better than standard preparative bead-based resins. Consequently, to achieve the same level of separation on a membrane sorber, a bed height 10-fold less can be utilized. Bed lengths of 1-5 mm are standard for membrane sorbers, compared to bed heights of 10-30 cm for bead-based systems. Due to the extreme column aspect ratios required for large-volume membrane sorbers, device design is critical. To maintain the inherent performance advantages associated with membrane sorbers, proper inlet and outlet distributors are required to efficiently and effectively utilize the available membrane volume. Membrane sorber technology is ideally suited for this application. Current commercial membrane sorbers, however, suffer from various drawbacks, including low binding strength, difficulty in removing viruses, endotoxins and nucleic acids.
A membrane sorber is a highly porous, interconnected media that has the ability to remove (ad- and/or absorb) some components of a solution when the latter flows through its pores. The properties of the membrane sorber and its ability to perform well in the required application depend on the porous structure of the media (skeleton) as well as on the nature of the surface that is exposed to the solution. Typically, the porous media is formed first, from a polymer that does not dissolve or swell in water and possesses acceptable mechanical properties. The porous media is preferably a porous membrane sheet made by phase separation methods well known in the art. See, for example, Zeman L J, Zydney A L, Microfiltration and Ultrafiltration: Principles and Applications, New York: Marcel Dekker, 1996. Hollow fiber and tubular membranes are also acceptable skeletons. A separate processing step is usually required to modify the external or facial surfaces and the internal pore surfaces of the formed porous structure to impart the necessary adsorptive properties. Since the membrane structure is often formed from a hydrophobic polymer, another purpose of the surface modification step is also to make the surfaces hydrophilic, or water-wettable.
There exist a number of approaches to modify the external or facial surfaces and the internal pore surfaces of a membrane. Those skilled in the art will readily recognize exemplary methods involving adsorption, plasma oxidation, in-situ free-radical polymerization, grafting and coating. The majority of these methods lead to formation of monolayer-like structures on the membrane surface, which most of the time achieve the goal of making it hydrophilic, yet fail to impart acceptable adsorptive properties, for example high capacity for the adsorbate. The capacity is defined as the amount (weight) of the adsorbate that can be retained by a given volume of the media. As long as all adsorption occurs on the membrane surface, the capacity will be limited by the membrane surface area. By their nature, microporous membranes have lower surface area compared to chromatography beads. One way to increase surface area is to reduce pore size, which obviously leads to significant losses in flux. For example, the maximum (monolayer) adsorption of protein on a 0.65 um polyethylene membrane (Entegris Corp, Billerica Mass.) is about 20 mg/ml, regardless of the type of surface interactions. This is significantly less than, for example, agarose chromatography beads, with typical capacity about 80 mg/ml.
The type of surface interactions driving the adsorption is defined by the specific application in which a given membrane sorber product is used. Currently, there is a need for a high-capacity, high-affinity sorber that removes viruses, nucleic acids, endotoxins, and host cell proteins (HCPs) from a solution of monoclonal antibodies (MABs). These impurities tend to have a lower isoelectric point than the MABs, which means that at a certain pH they will be negatively charged while the MAB will be positively charged. An anion exchanger, i.e. a media that bears a positive charge and attracts anions, is required to remove these impurities. A number of chemical moieties bear a positive charge in an aqueous solution, including primary, secondary, and tertiary amines, as well as quaternary ammonium salts. The amines are positively charged at pH below 11, while the ammonium salts bear the positive charge at all pH, so these groups are commonly called weak and strong anion exchangers, respectively.
Anion exchange membranes have multiple positively charged binding sites that attract and hold various impurities and contaminants. The amount of impurities that can be potentially removed is a function of the concentration of these binding sites on the membrane, and the chemical nature of the ligand (as well as the concentration of these ligands) is responsible for the strength of binding for the various impurities. High strength of binding is a key attribute for increasing the removal of impurities, for example, host cell proteins. Strength of binding (SB) is related to the ionic strength of solutions required to elute the bound impurities. SB of membrane sorber (measured in conductivity units, mS/cm) is determined as follows. First, a small amount of adsorbate solution is passed through the membrane sorber so the adsorbate binds to the membrane sorber. Second, the membrane sorber is eluted with increasing gradient of inorganic salt, such as sodium chloride. The minimum conductivity of elution solution required to elute off the adsorbate is recorded and defined as the SB of that membrane sorber. By increasing the sorber strength of binding, negatively charged impurities can be made to bind irreversibly to the membrane sorber, thereby significantly increasing the removal efficiency. This is particularly important for the removal of weakly bound populations of host cell proteins from an antibody stream.
Conventional flow-through anion exchangers typically contain a quaternary ammonium ligand that is responsible for attracting and binding negatively charged impurities, while repelling the positively charged product molecules. Conventional wisdom dictates that in order to bind impurities strongly by charge interaction, a strong anion exchanger, i.e., one that has a positive charge at all pH values, is necessary. The present invention demonstrates otherwise. The present inventors found that polymeric primary amines, preferably aliphatic polymers having a primary amine covalently attached to the polymer backbone, more preferably having a primary amine covalently attached to the polymer backbone by at least one aliphatic group, preferably a methylene group, bind negatively charged impurities exceptionally strongly and thus are the preferred class of materials for creating the adsorptive hydrogel on the surface of a membrane sorber.
Monoclonal antibodies continue to gain importance as therapeutic and diagnostic agents. The process of screening hybridoma libraries for candidate mABs is both time consuming and labor intensive. Once a hybridoma cell line expressing a suitable mAB is established, a purification methodology must be developed to produce sufficient mAB for further characterization. A traditional method for purifying involves using Protein A or Protein G affinity chromatography. The purified antibody is desalted and exchanged into a biological buffer using dialysis. The entire process typically requires several days to complete and can be particularly onerous if multiple mABs are to be evaluated in parallel.
U.S. Pat. No. 6,090,288 teaches preparation of amino group containing chromatography media for separation of peptides and nucleic acids. It is disclosed that a higher ionic strength is necessary for elution of proteins from weak anion exchange ligands vs. strong ones. The important structural feature of the separation media disclosed is that “at a distance of 2 or 3 atoms away from an amino nitrogen there is a hydroxyl group or a primary, secondary or tertiary amino group”. Herein, for example, we show that a loosely crosslinked coating of pure polyallylamine polymer does not require any additional groups to promote higher strength of binding of proteins.
U.S. Pat. No. 5,304,638 teaches using a protein separation medium that comprises a water-insoluble matrix carrying a plurality of polyamine groupings. One of the examples demonstrates preparing a polyallylamine surface modified agarose chromatography gel. However, the inventors of U.S. Pat. No. 5,304,638 fail to recognize the importance of using a primary amines vs. secondary and tertiary amines. No effort is made or described concerning controlling coating thickness to optimize sorption, or to crosslink the coating for stability. They emphasize the preferred number of carbon atoms between pairs of nitrogen atoms as being not more than 3. They introduce an empirical function Q, which is calculated based on the structure of polyamine grouping and pH and has a preferred value of at least 1.5. In polyallylamine, there are 5 carbon atoms between nearest nitrogen atoms, and the Q function for it would be close to 0.1.
U.S. Pat. No. 5,547,576 teaches preparing a porous membrane that has an immobilized polyamine coating and could be used to remove viruses from aqueous solution. The coating preparation involves first grafting a radical on the surface of the membrane and then reacting the radical with a polyamine compound. Grafting modifications are often impractical due to their inherent complexity: they are sensitive to the particular substrate a radical is grafted to, and can be expensive to implement in manufacturing environment. This method also suffers from the structural deficiencies discussed re: U.S. Pat. No. 5,304,638.
All three of these patents, U.S. Pat. Nos. 6,090,288, 5,304,638, and 5,547,576, fail to recognize the importance of control over the thickness of polyamine coating or the degree of polymer cross-linking within that coating. All of them rely on chemical reaction of an amine-containing compound with a reactive group that has been covalently immobilized on the surface, either by grafting or direct reaction. In the end of any such procedure, one is essentially limited to a monolayer-type amine-containing coating. High sorptive capacity of these membranes can only be achieved by increasing the surface area, as is the case with agarose chromatography beads. The present invention teaches that high sorptive capacity is achieved by building a relatively thick layer of loosely cross-linked polymeric layer on the membrane surface, a radically different approach from all those taught in the prior art.
Accordingly, it would be desirable to provide media and a flow-through anion exchanger including such media, that offer strong binding of impurities and that do not suffer from the shortcomings of the prior art. Such an exchanger is particularly useful in the purification of monoclonal antibodies from cell culture media using a chromatography scheme, when placed downstream of an affinity chromatography column that is optionally followed by one or more polishing steps carried out with a cation exchange column, for example.